How can I edit a shape-key mid-range without drivers?
Shape keys can be used to change a mesh shape with a certain amount of vertices into an other shape with more (or less) vertices. But for the sake of (my) argument lets assume it is the same amount of vertices for the start and end shape. The same way i think you are using shape keys.
In that case i believe a shape key can be seen as a recording of two location coordinates for every vertex in the mesh. (a recording like an animation keyframe). One for the start shape and one for the end shape.
I think (when the shapekey Value is changed from 0 to 1) an individual vertex moves from its starting position to its end position traveling the shortest possible distance. In other words, it moves in a straight line.
If i understand (or imagine your problem) correctly, you are essentially asking if a vertex (part of a single shapekey) can have a third coordinate location (like an animation keyframe) along the way. A third location it would be at if the Value is somewhere between 0 and 1, and that is not somewhere on the strait line between the start and end location. I'm pretty sure at the moment it can't.
Probably there is some other solution to your problem, but i think you would have to approach your problem (and possibly phrase your (new) question) differently.
Hope that gives some insight : )